Mail Activity Window always shows "Downloading Messages"
MacOS Mail now constantly shows "Downloading Messages" in the Activity Window (see below for an example).
This just started today (27 April), after my domain host migrated my email domain to a new server. Perhaps coincidentally, after that migration, my SpamSieve email "drone" setup on this Mac Mini always shows "Downloading Messages" in the Activity window.
The Connection Doctor shows all accounts working fine - green lights all down the line (see below for the results screen).
I have rebuilt each of the mailboxes on the machine, with no change.
I have re-booted the machine, stopped and started Mail (with and without network connectivity), and disabled / enabled each of the accounts in turn. Again, no change.
Help!
Mac Mini (mid 2011), running 10.11.6
<Image Edited by Host to Remove Personal Information>
Mac mini, OS X El Capitan (10.11.6), null